Variable speed voltage drive systems are used to vary the speed of motors, such as submersible motors used in submersible pumping systems deployed in wells. A typical submersible pumping system includes a pump and a motor, with the motor being electrically connected to a variable speed drive system over a cable that extends from the downhole location of the motor to an earth surface location of the variable speed drive system. The motor powers downhole components, such as the pump, to perform downhole tasks, such as to pump fluids from the downhole location to the earth surface. An example submersible motor is a three-phase induction-type motor. In the three-phase configuration, the variable speed drive system provides a three-phase input to the three-phase induction-type motor.
The load impedance of the cable and the downhole motor may cause resonance in signals from the variable speed drive system to the motor. The resonance is caused by undesirable harmonic components generated by the output of the drive system, which can cause voltage distortion and/or transients, zero-crossing noise, and other issues. To reduce resonance, a filter can be used to filter out harmonic components of each input signal from the variable speed drive system.
In some applications, the cable from the variable speed drive system to the downhole motor can be quite long, some as long as 25 kilometers or more. The long cable is associated with a large resistance that can cause a substantial voltage drop of each signal from the motor drive system along the cable. As a result, a separate step-up transformer (separate from the filter) typically has to be used to boost the voltage amplitude of an input signal from the variable speed drive system to compensate for the voltage drop along the cable. Use of separate units (a filtering unit and a voltage boost unit) to perform the filtering and amplitude boosting tasks may result in greater complexity and costs associated with deployment into a well of a submersible pump system, or other type of downhole system that includes a motor.